This invention relates to an ignition timing change-over device for internal combustion engines.
Various pollutants such as non-burnt hydrocarbons, carbon monoxide and nitrogen oxides, are produced in the course of combustion of fuel in an internal combustion engine. Retarding of ignition timing is known as an effective method for reduction of the amounts of the pollutants. So, a method of retarding the ignition timing over the entire operating range of an engine is commonly used in the art. However, this well-known method has a defect in that retardation of the ignition timing results in an undesirable reduction of the operation performance of the engine in the engine operating region in which the amounts of the production of the pollutants are relatively small. Various proposals have been made hitherto for obviating such defect by changing the ignition timing depending on the operating condition of the engine. One of such proposals intends to improve the starting performance of the engine, and at the same time, to reduce the amounts of the pollutants produced during idling. According to the proposal, the ignition timing is advanced before the upper dead point in the engine starting stage only, and is retarded after the upper dead point after the engine is started.
In studying the improvements on the reduction of engine performance due to advancing the ignition timing, the inventors of the present invention have found that the operating performance of the engine at a low temperature (that is, in a temperature range in which the engine cooling water is lower than 40.degree. C) can be improved by advancing the ignition timing more than that in a higher engine temperature range. A series of inventions based on the fact have been disclosed already.
Various other proposals have been made in an attempt to eliminate undesirable reductions of the engine operating performance due to the advanced ignition timing.
For example, U.S. Pat. No. 3,593,693 discloses an invention which aims to reduce the amounts of the pollutants produced during the running condition of a vehicle when the vehicle is coasting at a high speed and the throttle valve in the carburetor is closed in spite of the fact that the rotating speed of the engine is high. This prior art invention is featured by the fact that the ignition timing is advanced when the engine is rotating at a high speed in spite of the closure of the throttle valve, and the ignition timing is retarded subsequently when the speed of the engine is reduced to a level lower than a predetermined value.
U.S. Pat. No. 3,717,135 discloses an invention which intends to eliminate undesirable reductions of the operating performance in an engine in the starting stage as in the proposals above described. According to this prior art invention, the ignition timing is advanced during the engine starting stage and is then retarded after the engine is warmed to a temperature above a predetermined value so as to improve the engine starting performance, to ensure smooth operation of the engine during idling and to reduce production of the pollutants in a steady running condition.
Some of the above-mentioned proposals employ a dual point type contact breaker which operates independently of a known vacuum advancer. In some of the remaining proposals, a device is used in which a control mechanism responsive to the engine temperature is combined with a single point type contact breaker and a known vacuum advancer. In this latter device, the advancer is controlled depending on the combination of two variables, that is, the detected value of the engine temperature and the detected value of the load of the engine. Thus, this latter device is advantageous in that it can control the ignition timing more effectively and the size thereof is smaller than the devices of the other proposals.
The latter device above described is featured by the fact that the ignition timing is changed over the entire operating range of the engine in consideration of both the improvement in the engine operating performance and the reduction in the amounts of the pollutants discharged from the engine, and is used in combination with a known vacuum advancer. This latter device comprises a first valve responsive to the pressure at a point upstream of the throttle valve in the carburetor, a second valve responsive to the pressure at a point downstream of the carburetor throttle valve, and restraining means for restraining or releasing the valve rods of the first and second valves depending on the temperature of the engine. According to this device, the known vacuum advancer responds to the pressure on the downstream side of the carburetor throttle valve when the speed of the engine is increasing, and the pressure on the upstream side of the carburetor throttle valve is supplied into the vacuum advancer to render the vacuum advancer inoperative, or the vacuum advancer is advanced a suitable angle determined by a pressure intermediate the pressures on the upstream and downstream sides of the carburetor throttle valve when the steady state is reached and the engine is rotating at a constant speed. However, this device is also defective in that the ignition timing cannot be changed over all the operating conditions of the engine due to the fact that the ignition timing cannot be retarded to suit a specific operating condition of the engine. The present invention obviates such a defect.